In general, classification can be described as a method of separating mixtures of e.g. particles into two or more products. It is known to use hydrocyclones for performing classification of particulate material and they have proven very efficient at fine separation sizes. Often, hydrocyclones comprise a cylindrical top portion into which a slurry containing the particles to be classified is fed and an overflow is provided at the top of the cylindrical top portion. Attached to a lower end of the top portion is a conically shaped vessel which is open at its smallest end. The slurry is typically fed tangentially or in a volute path to the outer wall of the top portion, thus creating a whirling stream of the slurry which stream follows a path of gradually decreasing radius toward a point close to the narrowest radius of the cone, commonly known as the apex. As the spiral path approaches the apex of the hydrocyclone, a portion of it turns and begins to flow towards the opposite end, i.e. towards the cylindrical section. Also this flow is in a spiral path but of a radius smaller than the radius of the first spiral while rotating in the same direction. Thus a vortex is generated within the hydrocyclone. The pressure will be lower along the central axis of the vortex and increase radially outwardly towards the outer wall of the hydrocyclone. The idea is that the hydrocyclone will separate the particles of the slurry according to shape, size and specific gravity with faster settling particles moving towards the outer wall of the hydrocyclone eventually leaving the hydrocyclone through the underflow. Slower settling particles will move towards the central axis and travel upwardly, eventually leaving the hydrocyclone through a discharge tube (overflow). The discharge tube is normally extending down into the cylindrical section such that short-circuiting of the feed is prevented. This is also known as a “vortex finder”. This separation according to shape, size and specific gravity is sometimes denominated “stratification”. However, this stratification of the material is not always fully achieved thus causing an incomplete classification.